Increased running speeds of paper and board machines provide new problems to be solved, which problems are mostly related to the runnability of the machine. Currently running speeds of up to about 1600 meters per minute are employed in paper machines. At these running speeds, the so-called closed press sections, which comprise a compact combination of press rolls arranged around a smooth-faced center roll, for the most part still operate satisfactorily. As examples of these press sections, reference is made to the current assignee's Sym-Press II.TM. and Sym-Press O.TM. press sections.
It is a recognized principle in the art that dewatering taking place by pressing is more advantageous than dewatering by evaporation from the point of view of energy economy. For this reason, attempts are made to remove a maximal amount of water out of the web by pressing, in order that the proportion of water to be removed by evaporation can be made as low as possible. Increased running speeds of paper and board machines, however, provide new, so far unsolved problems expressly for dewatering taking place by pressing because the press impulse applied by such dewatering by pressing, e.g., in press nips, cannot be increased sufficiently by the prior art means, above all because at high speeds the nip times remain insufficiently short and, on the other hand, the peak pressure of the compression in the nip cannot be increased beyond a certain limit without destroying the structure of the web.
With increasing running speeds of paper machines, the problems of runnability of a paper machine are also manifested with higher emphasis, because a web with a high water content and low strength does not endure an excessively high and sudden compression pressure impulse or the dynamic forces produced by high speeds, but rather web breaks and other disturbances in the operation of the paper machine arise and cause standstills. In modern paper machines, the cost of standstill time is today about 50,000 Finnish Marks (FIM) per hour (roughly $11,000 which adds up to significant amounts).
Further drawbacks of the prior art wire parts and press sections include the requirement of suction energy of the suction rolls commonly used in them and the noise problems arising from suction rolls. Moreover, suction rolls with their perforated mantles, inner suction boxes, wearing seals, and other suction arrangements are components with a high cost and which require repeated servicing and consume an abundance of energy. As an example, it can be mentioned that in a board machine having a width of about 6 meters, the cost of suction energy of one suction roll is about 1 million FIM per year ($220,000). In addition to the drawbacks mentioned above, the efficiency of the prior art suction rolls is lowered significantly at particularly high web speeds, because the suction force does not have sufficient time to act upon the web in the intended manner through the long perforations in the relatively thick mantle of the suction roll.
In the prior art press sections, the web is often passed from the forming wire into the first press nip on a pick-up felt, which also operates as a press fabric that receives significant amounts of water in the first press nip, which is either a roll nip or an extended nip. In the first press nip, it is often necessary to employ a relatively high compression pressure and to deal with large quantities of water, and it is one of the drawbacks arising from this that the outer face of the press felt tends to be contaminated and its porous fibrous structure tends to be partially blocked. Attempts are made to prevent this blockage by means of efficient felt conditioning devices, which are, however, quite expensive, spacious components which consume an abundance of energy.
Recently, even speeds as high as about 40 meters per second (2400 meters per minute) have been contemplated as speeds of printing-paper machines. Applications at speeds as high as this, in particular in wide machines, provide ever more difficult problems to be solved, of which problems the most important ones are runnability and adequate dewatering capacity of the machine at a high web speed. Similarly, in board machines (basis weight of the web being greater than about 100 grams per square meter), attempts are made to increase the present web speeds (about 8 to about 15 meters per second) to the level of from about 15 to about 25 meters per second.
Important drawbacks of the press felts used in the prior art press sections include the effect of rewetting the web and the tendency of contamination because, in particular when the press felts run through a high-pressure nip or nips, particles of contaminants tend to be affixed and to adhere to the press fabrics. For this reason, the operation of the press fabrics is disturbed and their cleaning requires efficient conditioning devices, which consume a considerable amount of energy.
Moreover, in high-pressure press nips, the prior art porous press felts are subjected to intensive wear and strain, so that the felts must be replaced rather frequently, which increases the costs to a considerable extent.
With respect to the prior art most closely related to the present invention, the following is stated.
In conventional board machines, a pre-press provided with a fabric circulation of its own has been employed, in which pre-press the linear load is for wires (so-called wire press) of an order of from about 15 kN/m to about 20 kN/m and for press felts from about 40 kN/m to about 50 kN/m. Experience of operation of such conventional board machines has been obtained from wire presses in particular with paper grades having a basis weight higher than about 80 grams per sq.m. Moreover, several different presses operating by means of a pick-up suction roll have been in use, for example, in machines that produce kraft paper. With respect to these and to the rest of the prior art closely related to the present invention, reference is made to the current assignee's Finnish Patent Application No. 905798 and to the corresponding European Patent Application Publication No. 0 487 483 A1 and U.S. Pat. No. 5,389,205 (which is hereby incorporated by reference herein). In FIGS. 6A, 6B and 6C in these applications and the U.S. patent, the use of a so-called wire press nip is illustrated, by means of which wire press nip arranged in connection with the web, the dry solids content of the web is increased from about 10% to about 20%. The wire nips are preferably intended to be nips that remove water in two directions, either as a roll nip provided with two opposite press fabrics (FIG. 6A in these publications), an extended nip provided with an upper press felt (FIG. 6B), or a belt-tensioned nip in which there is an upper press fabric (FIG. 6C), i.e., both web-engaging press fabrics are significantly water-receivable. After the wire nips, the pre-pressed web is passed to the respective pick-up points where it is transferred by means of the suction of the pick-up roll to the lower face of an upper pick-up press felt and then carried thereon into the next nip, which is either an extended nip or a roll nip.
A wire nip arrangement substantially similar to that described above is also described in International Patent Application WO 94/29519 (applicants Valmet-Tampella Inc.), to which publication, reference is made in respect of the prior art.
In the prior art wire presses, it has generally been considered necessary that the dewatering takes place in the wire nips in two directions, i.e., also toward the upper press fabric. An exception from this generality consists of what is called lump breakers, which are used in board machines in the manner known from the prior art and which can also be used without a press fabric. As is known from the prior art, a lump breaker is placed in connection with a wire suction roll to form a wire nip, which increases the dry solids content of the web by just a few percentage units, and the primary function of this roll is to improve the upper surface properties of the board web and to facilitate the threading of the web. Most often, as lump breakers, a smooth roll provided with a resilient rubber coating is used, whose diameter is about 600 to about 800 mm, and the linear load in the nip is maximally about 30 kN/m.
Further, with respect to the prior art related to the present invention, reference is made to European Patent Application Publication No. 0 359 696 A2 in the name of Beloit Corp., in which a roll nip placed in connection with a forming wire is described, which nip is provided with two press felts so that the lower press felt is arranged around a lower press roll situated inside the forming-wire loop and the upper press-suction roll is arranged inside the upper-feet loop. On the upper press-suction roll, the web is transferred from the forming wire onto the lower face of the water-receiving press felt and thereon, further as a horizontal run into the first extended nip, through which the upper press felt runs while it also operates as a press fabric in that nip. In the press sections mentioned above, even if objectives similar to those of the present invention are partly achieved in them, the press-suction roll can, however, not be eliminated, nor can rewetting of the web or the tendency of wear and contamination of the press felt be eliminated, which phenomena are particularly significant drawbacks expressly in press section similar to that described in EP 0 359 696.